1. Field of the Invention
This invention relates generally to electrochemical cells in which lithium is the active species, and more particularly to high temperature electrochemical cells having a molten salt electrolyte.
2. Prior Art
There is a great deal of current interest in better methods for energy storage. This is especially important for applications such as electric vehicles and the large scale storage of electric energy to level the load of stationary power plants. It does not appear that major improvements in important parameters, such as the amount of energy stored per unit weight or volume, at reasonable costs, can be expected from systems that employ aqueous electrolytes.
One of the developments currently being pursued involves a lithium-based cells, since lithium is very electropositive and light weight. In such cells, the negative electrode is a lithium alloy (typically either lithium-aluminum or lithium-silicon), the positive electrode is an iron sulfide, and the electrolyte is a molten salt, such as the eutectic composition in the lithium chloride-potassium chloride system. Because of the high melting point of such salts, operation is normally in the temperature range of about 350-500 degrees centigrade.
Various degradation processes, such as corrosion of the cell container, seals, and other components, are accelerated by such high temperatures. Sulfides are highly reactive under the high temperatures required, and use of iron sulfides as the cathode material is a major factor in the high corrosion incurred in these lithium-based cells. Additionally, it is believed that potassium ions from the molten salt electrolyte tend to compete with lithium ions at iron sulfide cathodes. The potassium ions may then form reaction products in the positive electrode which decompose slowly and reduce the current density.